1. Field of the Invention
The present invention relates to a fluid-filled bladder suitable for footwear applications. The invention concerns, more particularly, a fluid-filled bladder having a tensile member with flexion areas that enhance the overall flexibility of the bladder.
2. Description of Background Art
A conventional article of athletic footwear includes two primary elements, an upper and a sole structure. The upper provides a covering for the foot that securely receives and positions the foot with respect to the sole structure. In addition, the upper may have a configuration that protects the foot and provides ventilation, thereby cooling the foot and removing perspiration. The sole structure is secured to a lower surface of the upper and is generally positioned between the foot and the ground. In addition to attenuating ground reaction forces and absorbing energy (i.e., imparting cushioning), the sole structure may provide traction and control foot motion, such as over pronation. Accordingly, the upper and the sole structure operate cooperatively to provide a comfortable structure that is suited for a wide variety of ambulatory activities, such as walking and running. The general features and configuration of the sole structure are discussed in greater detail below.
The sole structure of athletic footwear generally exhibits a layered structure that includes a comfort-enhancing insole, a resilient midsole formed from a polymer foam, and a ground-contacting outsole that provides both abrasion-resistance and traction. Suitable polymer foam materials for the midsole include ethylvinylacetate or polyurethane that compress resiliently under an applied load to attenuate ground reaction forces and absorb energy. Conventional foam materials are resiliently compressible, in part, due to the inclusion of a plurality of open or closed cells that define an inner volume substantially displaced by gas. That is, the foam includes bubbles formed in the material that enclose the gas. Following repeated compressions, however, the cell structure may deteriorate, thereby resulting in decreased compressibility of the foam. Thus, the force attenuation and energy absorption characteristics of the midsole may decrease over the lifespan of the footwear.
One way to overcome the drawbacks of utilizing conventional foam materials is disclosed in U.S. Pat. No. 4,183,156 to Rudy, hereby incorporated by reference, in which cushioning is provided by inflatable inserts formed of elastomeric materials. The inserts include a plurality of tubular chambers that extend substantially longitudinally throughout the length of the footwear. The chambers are in fluid communication with each other and jointly extend across the width of the footwear. U.S. Pat. No. 4,219,945 to Rudy, hereby incorporated by reference, discloses an inflated insert encapsulated in a foam material. The combination of the insert and the encapsulating material functions as a midsole. An upper is attached to the upper surface of the encapsulating material and an outsole or tread member is affixed to the lower surface.
Such bladders are generally formed of an elastomeric material and are structured to have an upper or lower surface that encloses one or more chambers therebetween. The chambers are pressurized above ambient pressure by inserting a nozzle or needle connected to a fluid pressure source into a fill inlet formed in the bladder. After the chambers are pressurized, the fill inlet is sealed, for example, by welding, and the nozzle is removed.
Bladders of this type have been manufactured by a two-film technique, in which two separate sheets of elastomeric film are formed to exhibit the overall peripheral shape of the bladder. The sheets are then welded together along their respective peripheries to form a sealed structure, and the sheets are also welded together at predetermined interior areas to give the bladder a desired configuration. That is, the interior welds provide the bladder with chambers having a predetermined shape and size at desired locations. Such bladders have also been manufactured by a blow-molding technique, wherein a liquefied elastomeric material is placed in a mold having the desired overall shape and configuration of the bladder. The mold has an opening at one location through which pressurized air is provided. The pressurized air forces the liquefied elastomeric material against the inner surfaces of the mold and causes the material to harden in the mold, thereby forming a bladder with the desired shape and configuration.
Another type of prior art bladder suitable for footwear applications is disclosed in U.S. Pat. Nos. 4,906,502 and 5,083,361, both to Rudy, and both hereby incorporated by reference. This type of bladder is formed as a fluid pressurized and inflated structure that comprises a hermetically sealed outer barrier layer which is securely fused substantially over the entire outer surfaces of a tensile member having the configuration of a double-walled fabric core. The tensile member is comprised of first and second outer fabric layers that are normally spaced apart from one another at a predetermined distance. Connecting or drop yarns, potentially in the form of multi-filament yarns having many individual fibers, extend internally between the proximal or facing surfaces of the respective fabric layers. The filaments of the drop yarns form tensile restraining means and are anchored to the respective fabric layers. A suitable method of manufacturing the double walled fabric structure is double needle bar Raschel knitting.
U.S. Pat. Nos. 5,993,585 and 6,119,371, both issued to Goodwin et al., and both hereby incorporated by reference, disclose a bladder utilizing a tensile member, but without a peripheral seam located midway between the upper and lower surfaces of the bladder. Instead, the seam is located adjacent to the upper surface of the bladder. Advantages in this design include removal of the seam from the area of maximum sidewall flexing and increased visibility of the interior of the bladder, including the connecting yarns. The process utilized to form a bladder of this type involves the formation of a shell, which includes a lower surface and a sidewall, with a mold. A tensile member is placed on top of a covering sheet, and the shell, following removal from the mold, is placed over the covering sheet and tensile member. The assembled shell, covering sheet, and tensile member are then moved to a lamination station where radio frequency energy fuses opposite sides of the tensile member to the shell and covering sheet and fuses a periphery of the shell to the covering sheet. The bladder is then pressurized by inserting a fluid so as to place the connecting yarns in tension.
While the cushioning benefits of bladders in articles of footwear are well documented, the prior art bladders with a tensile member having the configuration of a double-walled fabric core are generally considered to be relatively inflexible. The present invention relates, therefore, to a more flexible fluid-filled bladder with a tensile member.